1. Field of the Invention
This invention relates generally to the field of protein kinases, oncogenes and oncoproteins and, specifically, to a protein kinase which binds, phosphorylates and potentiates the c-Jun N-terminal activation domain.
2. Description of Related Art
A number of viral and cellular genes have been identified as potential cancer genes, collectively referred to as oncogenes. The cellular homologs of viral oncogenes, the proto-oncogenes or c-oncogenes, act in the control of cell growth and differentiation or mediate intracellular signaling systems. The products of oncogenes are classified according to their cellular location, for example, secreted, surface, cytoplasmic, and nuclear oncoproteins.
Proto-oncogenes which express proteins which are targeted to the cell nucleus make up a small fraction of oncogenes. These nuclear proto-oncoproteins typically act directly as transactivators and regulators of RNA and DNA synthesis. Nuclear oncogene products have the ability to induce alterations in gene regulation leading to abnormal cell growth and ultimately neoplasia. Examples of nuclear oncogenes include the myc, ski, myb, fos and jun genes.
The c-Jun protein, encoded by the c-jun proto-oncogene, is an important component of the dimeric, sequence specific, transcriptional activator, AP-1. Like other transcriptional activators, c-Jun contains two functional domains, including a DNA binding domain and a transactivation domain. The DNA binding domain is located at the C-terminus and is a BZip structure which consists of conserved basic (B) and leucine zipper (Zip) domains that are required for DNA binding and dimerization, respectively. The N-terminus contains the transactivation domain. Although c-Jun expression is rapidly induced by many extracellular signals, its activity is also regulated post-translationally by protein phosphorylation. Phosphorylation of sites clustered next to c-Jun's DNA binding domain inhibits DNA binding (Boyle, et al., Cell, 64:573, 1991; Lin, et al., Cell, 70:777, 1992). Phosphorylation of two other sites, Ser 63 and Ser 73, located within the transactivation domain, potentiates c-Jun's ability to activate transcription (Binetruy, et al., Nature 351:122, 1991; Smeal, et al., Nature 354:494, 1991). Phosphorylation rates of these sites are low in non-stimulated cells and are rapidly increased in response to growth factors such as platelet derived growth factor (PDGF) or v-Sis, or expression of oncogenically activated Src, Ras and Raf proteins. In myeloid and lymphoid cells, phosphorylation of these sites is stimulated by the phorbol ester, TPA, but not in fibroblasts and epithelial cells. These differences may be due to different modes of Ha-ras regulation in lymphoid cells versus fibroblasts.
Many proteins cooperate with each other in the activation of transcription from specific promoters. Through this cooperation, a gene can be transcribed and a protein product generated. Members of the Fos proto-oncogene family, along with members of the Jun gene family, form stable complexes which bind to DNA at an AP-1 site. The AP-1 site is located in the promoter region of a large number of genes. Binding of the Fos/Jun complex activates transcription of a gene associated with an AP-1 site. In cells that have lost their growth regulatory mechanisms, it is believed that this Fos/Jun complex may "sit" on the AP-1 site, causing overexpression of a particular gene. Since many proliferative disorders result from the overexpression of an otherwise normal gene, such as a proto-oncogene, it would be desirable to identify compositions which interfere with the excessive activation of these genes.
For many years, various drugs have been tested for their ability to alter the expression of genes or the translation of their messages into protein products. One problem with existing drug therapy is that it tends to act indiscriminately and affects healthy cells as well as neoplastic cells. This is a major problem with many forms of chemotherapy where there are severe side effects primarily due to the action of toxic drugs on healthy cells.
In view of the foregoing, there is a need to identify specific targets in the abnormal cell which are associated with the overexpression of genes whose expression products are implicated in cell proliferative disorders, in order to decrease potential negative effects on healthy cells. The present invention provides such a target.